Add npm publication gates to postgresai release process

Summary

• Correct npm dist-tag expectations: final tags move latest; prereleases move only rc, beta, or dev.
• Add pgai package checks alongside postgresai.
• Add dist-tag verification and fresh npx / bunx entrypoint checks for pre-release and GA validation.

Context

Nik flagged that the postgresai release process needs explicit npm publication control before 0.15 GA.

Validation

• Reviewed rendered diff locally.
• Pipeline passed: https://gitlab.com/postgres-ai/rules/-/pipelines/2561004969

Closes #4 (closed)